Valve lifters or tappets in some internal combustion engines, especially high speed automotive engines, use rollers that ride in contact with the cams of cam shafts that operate the valve lifters; i.e., that reciprocate the lifters, which are substantially cylindrical, in cylindrical bores of the engine block. Lifters are typically aligned in a row along a cam shaft and are lubricated by a straight and common transverse oil passage that intersects the bores, either centrally or tangentially. Serial flow of oil through successive bores is facilitated by a peripheral groove in each lifter body. The portion of the body above and below the groove prevents loss of lubricating oil through the open ends of the bores in which the lifters reciprocate and extend.
With current internal combustion engines, a typical valve lifter uses needle bearings to journal a roller cam follower on a pin or axle. This type of arrangement provides a minimal amount of lubrication to the needle bearings through the typical valve lifter oil passage which intersects the bores. In engines used for high speed automobiles, such as racing automobiles, this amount of lubrication provided to the needle bearings is inadequate and leads to early failure of the needle bearings.
A prior attempt at improving the life of needle bearings in high speed automotive engines utilized a secondary oil passage. However, the outlet was merely in the general area of the needle bearings and therefore did not provide optimum lubrication to the needle bearings. Another prior attempt at improving lubrication of the needle bearings moved the opening of a secondary oil passage closer to the axle or pin associated with the roller.
Needle bearings, even when adequately lubricated, tend to fail after short use under high valve spring loads when engines are operated at high rpm--two conditions that are prevalent in high speed automotive engines. Other bearing constructions, however, have not been deemed commercially suitable for such uses, although stronger plane bearings have been used in larger low speed engines and have been lubricated with an oil passage extending into a pin or axle associated with the roller. In such a system three passages have been drilled within the pin or axle, which is made of bronze and therefore is relatively soft, making it is easy to drill lubrication passages. An axial, horizontal passage is drilled along the axis of the pin, beginning at a first end and stopping just short of an opposite end. A vertical supply passage is drilled at the opposite end and begins on an outer cylindrical surface of the pin and ends at the axial passage to form an elongated L-shape. A second horizontal passage is drilled transverse to the axial passage and opens on opposite sides of the outer cylindrical surface of the pin to provide lubrication to the bearing. The axial passage must be plugged at the first end to prevent oil from escaping. With a high speed engine, however, such a construction will not work. The soft metal of the plane-bearing axle is not suitable for the wear experienced by the bearings of a lifter in a high speed engine. In addition, in a high speed engine the components of the engine, and therefore the valve lifters themselves, are much smaller. Therefore, in addition to having to drill down a small lifter body in order to drill the oil passage to communicate with the pin, one must also attempt to distribute the oil to the bearing surface, which is a more formidable task with a small bearing pin of hard wear-resistant metal, as would be required for this type of use.